Ion implanters are widely used in semiconductor manufacturing to selectively alter conductivity of materials. In a typical ion implanter, ions generated from an ion source are directed through a series of beam-line components which include one or more analyzing magnets and a plurality of electrodes. The analyzing magnets select desired ion species, filter out contaminant species and ions having undesirable energies, and adjust ion beam quality at a target wafer. Suitably shaped electrodes may modify the energy and the shape of an ion beam.
In production, semiconductor wafers are typically scanned with an ion beam. As used hereinafter, “scanning” of an ion beam refers to the relative movement of an ion beam with respect to a wafer or substrate surface.
An ion beam is typically either a “spot beam” having an approximately circular or elliptical cross section or a “ribbon beam” having a rectangular cross section. For the purpose of the present disclosure, a “ribbon beam” may refer to either a static ribbon beam or a scanned ribbon beam. The latter type of ribbon beam may be created by scanning a spot beam back and forth at a high frequency.
In the case of a spot beam, scanning of a wafer may be achieved by sweeping the spot beam back and forth between two endpoints to form a beam path and by simultaneously moving the wafer across the beam path. Alternatively, the spot beam may be kept stationary, and the wafer may be moved in a two-dimensional (2-D) pattern with respect to the spot beam. In the case of a ribbon beam, scanning of a wafer may be achieved by keeping the ribbon beam stationary and by simultaneously moving the wafer across the ribbon beam. If the ribbon beam is wider than the wafer, a one-dimensional (1-D) movement of the wafer may cause the ribbon beam to cover the entire wafer surface. The much simpler 1-D scanning makes a ribbon beam a desired choice for single-wafer ion implantation production.
However, ribbon beams, as well as spot beams, can suffer from intrinsic non-uniformity problems. For example, a ribbon beam may consist of a plurality of beamlets, wherein each beamlet may be considered, conceptually, as one spot beam. Although beamlets within a ribbon beam are directed in the same general direction, any two beamlets may not be directed in exactly the same direction. In addition, each beamlet may have an intrinsic angle spread. As a result, during ion implantation with a ribbon beam, different locations on a target wafer may experience different ion incident angles. Furthermore, the beamlets may not be evenly spaced within the ribbon beam. One portion of the ribbon beam where beamlets are densely distributed may deliver a higher ion dose than another portion of the ribbon beam where beamlets are sparsely distributed. Therefore, a ribbon beam may lack angle uniformity and/or dose uniformity.
Ion beam angle uniformity and/or dose uniformity may be controlled by several ion implantation components. For example, electric and/or magnetic elements may be utilized.
FIG. 1 shows a conventional ion implanter 100 comprising an ion source power supply 101, an ion source 102, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet analyzer 110, and a second deceleration (D2) stage 112. The D1 and D2 deceleration stages (also known as “deceleration lenses”) may each comprise multiple electrodes with a defined aperture to allow an ion beam 10 to pass therethrough. By applying different combinations of voltage potentials to the multiple electrodes, the D1 and D2 deceleration lenses may manipulate ion energies and cause the ion beam 10 to hit a target workpiece 114 at a desired energy. A number of measurement devices 116 (e.g., a dose control Faraday cup, a traveling Faraday cup, or a setup Faraday cup) may be used to monitor and control characteristics of the ion beam 10. The above-mentioned D1 or D2 deceleration lenses may be electrostatic triode (or tetrode) deceleration lenses.
Significant changes in ion energies that take place in the electrostatic deceleration lenses can have a substantial impact on a shape of the ion beam 10. For example, space charge effects are more significant in low-energy ion beams than in high-energy ion beams. Therefore, a considerable number of ions may be lost before they reach the target wafer as when using low-energy ion beams. As a result, the effective dose and angle uniformity of the ion beam 10 may be substantially reduced.
There have been several attempts to reduce the above-described space charge effects in electrostatic triode lenses. For example, tuning voltages of deceleration lenses may help reduce space charge effects. However, because forces associated with space charge effects may be highly non-linear (especially if the ion beam 10 is not elliptical), tuning voltages of deceleration lenses may be very challenging without accurate tuning assistance to compensate for space charge effects. Another example may include using one or more magnetic elements (e.g., corrector bars) at entrance and/or exit regions of a magnetic deflector to improve uniformity across a target wafer.
Although these additional electric and/or magnetic components have been utilized in conventional ion implanters to somewhat improve either angle uniformity and/or dose uniformity of an ion beam, a more efficient solution has yet to be made available for providing ion beams that meet current dose and angle uniformity requirements for ion implantation production. For example, it is typically required that a ribbon beam should produce, in a wafer plane, a dose uniformity with less than 1% variation together with an angle uniformity with less than 0.5° variation. Such stringent uniformity requirements are becoming more difficult to meet since both types of uniformity may be elusive, especially in semiconductor manufacturing which require relatively high specificity and reliability.
In view of the foregoing, it may be understood that there are significant problems and shortcomings associated with current ion implantation technologies.